1. Field of the Invention
The present invention relates to an image forming apparatus, and particularly one that includes a thermal fixing device for fixing an image formed on a recording medium according to an electrophotographic process that uses a laser-beam printer or the like as image forming means on the recording medium.
2. Description of the Related Art
Conventionally, facsimile apparatuses, each including a heat-roller-type thermal fixing device in which a toner image formed on a recording sheet according to an electrophotographic process is fixed on the recording sheet by a heated roller, have been widely used. FIG. 3 is a cross-sectional view of a principal portion of a thermal fixing device of a conventional facsimile apparatus.
In FIG. 3, a halogen-lamp heater 1 (hereinafter abbreviated as heater 1), serving as a heating member, is disposed at a central portion of a fixing roller 2 made of a metallic material having a thickness of about 1 mm. A thermistor 7 is brought in contact with the surface of the fixing roller 2. The thermistor 7 detects the surface temperature of the fixing roller 2, and transmits a detected-temperature signal to control means (not shown). The control means controls the surface temperature of the fixing roller 2 at a predetermined fixing temperature by controlling electric power supply to the heater 1 based on the detected-temperature signal of the thermistor 7. Reference numeral 4 represents a pressing roller. A recording sheet P having a toner image formed thereon is guided by a guide member 5 to a pressing position 6 between the pressing roller 4 and the fixing roller 2, and the toner image on the recording sheet P is heated and fixed by the fixing roller 2 heated to the predetermined temperature by the heater 1. The recording sheet P having the fixed image thereon is conveyed to sheet discharging rollers 10 by guide members 9 and is discharged to the outside of the apparatus.
It is impossible to instantaneously raise the temperature of the heater 1 and the fixing roller 2 heated by the heater 1 to the predetermined temperature. Hence, in order to fix the toner image, a time is required to raise the temperature of the thermal fixing device from its ambient temperature when there is no power supply to the predetermined fixing temperature, i.e., to perform so-called temperature rise control. Facsimile apparatuses also having the function of copying documents have a plurality of recording modes, i.e., a reception mode for recording an image received by the apparatus and a copy mode for copying a document. The above-described temperature rise control for thermal fixing device for fixing a formed image is started at a constant timing in any one of the recording modes. In the copy mode, the temperature rise control of the thermal fixing device is started when the user operates a start key for starting a copying operation, for example, after setting an original on the apparatus.
In the reception mode, the user is not typically waiting for the discharged recording sheet after automatically fixing an image when after image has been received. Hence, the necessity of shortening the time from image reception until first printing and the discharge of the recording sheet is not as great. On the other hand, in the copy mode in which the user directly operates the apparatus in order to copy a document, it is desired to perform a copying operation as fast as possible by shortening a first printing time from the operation of the start key to the discharge of the recording sheet. In the above-described conventional facsimile apparatus, however, since the temperature rise control of the thermal fixing device is performed according to a constant method irrespective of switching of the recording mode, such requirement cannot be dealt with.
In order to shorten the first printing time in all of the recording modes, a method of shortening the time to raise the temperature of the thermal fixing device to a predetermined fixing temperature by always preheating the thermal fixing device can be considered. This method, however, has the problems that electric power consumption during waiting increases, and that noise is generated by a fan that is needed for discharging heat to the outside of the apparatus.
It is an object of the present invention to provide an image forming apparatus which can shorten a first printing time in accordance with a recording mode without increasing electric power consumption.
In accordance with these objects, there is provided an image forming apparatus having a plurality of modes switchable by a user, the apparatus comprising image forming means for forming an unfixed image on a recording medium, thermal fixing means for fixing the unfixed image on the recording medium, means for switching recording modes, and temperature rise control means for controlling a fixing temperature of the thermal fixing means. The temperature rise control means begins electric power supply to the thermal fixing means to start temperature control and elevate the fixing means to the fixing temperature coincident with the switching of the mode by the user.
The foregoing and other objects, advantages and features of the present invention will become more apparent from the following description of the preferred embodiment taken in conjunction with the accompanying drawings.